*** Clarification about "Transparent PNG" pictures ***This image format is mostly used for web pages. It's less supported on operating systems (such as iOS) and most photography apps.

However, these images are fine when used in web-sites / inside some apps [for example: Apple "Pages" app for iOS] / desktop photo editor softwares / or submitting avatar pictures on web.

The iOS Photos app displays the transparent pixels as white / and sometimes with sudden color-change to black. For large-size pictures, the transparency would be vissible when you are playing (finger-zooming) with a thumbnail in photo gallery.

Another way to find transparency is to open the picture in "Circle Crop app" and then zoom-out with two fingers to view any transparent area.